Feasibility study on the use of irrigation as part of a long-term acid mine water management strategy in the Vaal Basin

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Protecting the environment from contaminated mine water decanting in increasing volumes from the goldfields presents an enormous challenge, probably greater than that of coal mine water because the decant points are in urban areas. Some of the water is acidic but most of it is partly neutralised through water-rock interaction. The main limitation of the water is an undesirable concentration of sulfate salts and metals, chiefly iron, and to a lesser extent manganese, aluminium and various trace elements. This project addressed some of the possibilities of irrigating land and producing crops with the mine water, primarily after it has been neutralised with lime but also as a form of land treatment in which the raw mine water is applied to soils or mine tailings that have been preconditioned with slaked lime or limestone to achieve in situ neutralisation and sequestration of many of the contaminants. Supplementary treatments were also explored using aluminium sulfate and locally mined ferromanganese wad, currently used for uranium recovery in the gold mines.
The project found it highly probable that goldfield mine water can be used cost-effectively to irrigate vegetation on mine tailings or salt tolerant crops such as wheat or soybean on agricultural land. Following irrigation the salts in the water become concentrated and the dominant ions, calcium and sulfate, precipitate as gypsum. It is calculated that about 60% of applied salts will be retained within the soil when irrigating with neutralised mine water. Crop model simulations estimate that irrigating with neutralised mine water can result in wheat yields of around 9 tonnes/ha and soybean yields of 5 tonnes/ha when grown in rotation. Even under worst case scenarios in which farmers have to pay for the infrastructure to deliver the mine water to their farms, an income of >R240 000/year can still be realised for a 40 hectare farm.
Clay soils and mine tailings have further capacity to retain many of the other salts present in the water. Results from this study indicate that 75-90% of salts can be removed when raw mine water is applied to mine tailings or clay soils. The use of aluminium sulfate – which works synergistically with lime – as part of the pre-treatment process potentially has several benefits, including smaller, more economical treatment plants.
Reverse osmosis has been proposed for treating mine water in the Vaal Basin but is expensive, energy intensive and, like other processes, leaves a saline residue which requires disposal. By contrast, irrigating with chemically treated water will enable its immediate productive use. The socio-economic benefits could be far reaching. In addition, South Africa currently is not well positioned to provide electricity for reverse osmosis and will need to accept the high carbon footprint indefinitely. A Life Cycle Assessment comparing conventional reverse osmosis with the irrigation option together with reverse osmosis of the smaller volumes of irrigation return flows demonstrated significantly lower impacts for the latter option for global warming potential, non-renewable resource (fossil fuel) depletion and acidification potential.
Following these positive results further research has been recommended, including the establishment of a pilot plant irrigating with actual purpose-treated mine water, exploring optimal geo-hydrological settings for land/irrigation treatment schemes and a thorough cost comparison with the conventional reverse osmosis option. The favourable implementation of this technology can have far reaching consequences, not only in the Witwatersrand goldfields, but also the Mpumalanga coalfields and many other regions around the world with a legacy of intensive mining.